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DNA Fingerprinting

VOCABULARY

DNA fingerprint

KeY CONCept DNA fingerprints identify people at the molecular level.

MAIN IDeAS A DNA fingerprint is a type of restriction map. DNA fingerprinting is used for identification.

Connect to Your World You hear about it in the news all the time. DNA evidence is used to convict a criminal, release an innocent person from prison, or solve a mystery. A couple of decades ago, the lines and swirls of someone’s fingertip were a detective’s best hope for identifying someone. Now, investigators gather biological samples and analyze DNA for another kind of evidence: a DNA fingerprint.

MAIN IDeA 6h

A DNA fingerprint is a type of restriction map.Unless you have an identical twin, your complete set of DNA, or your genome, is unique. This variation in DNA among people is the basis of DNA finger-printing. A DNA fingerprint is a representation of parts of an individual’s DNA that can be used to identify a person at the molecular level. A DNA fingerprint is a specific type of restriction map, which you learned about in Section 1. First, a DNA sample is cut with a restriction enzyme. Then the DNA fragments are run through a gel and the pattern of bands on the gel is analyzed. As you can see in figure 3.1, a DNA fingerprint can show relation-ships among family members. The children (C) have similar DNA fingerprints to one another, but they are not identical. Also, their DNA fingerprints are combinations of the DNA fingerprints of the parents (M and F). The greatest differences in DNA among people are found in regions of the genome that are not parts of genes. As a result, DNA fingerprinting focuses on noncoding regions of DNA, or DNA sequences outside genes. Noncoding DNA sequences often include stretches of nucleotides that repeat several times, one after another, as shown in figure 3.2. Each person’s DNA differs in the numbers of copies of the repeats. For example, one person may have seven repeats in one location, and another person may have three in the same place. To get to the specific regions of DNA that can be identified through DNA fingerprinting, the DNA is cut in known locations with restriction enzymes. The differences in the number of repeats are found by separating the DNA fragments with gel electrophoresis. When there are more repeats, a DNA fragment is larger. The pattern of DNA fragments on a gel represents the uniqueness of a person’s DNA. Individuals might have some of the fragments in common, but it is very unlikely that all of them would be the same.

Synthesize Does a DNA fingerprint show a person’s genotype? Why or why not?

(mother) (child 1) (child 2) (father)

figure 3.1 DNA fingerprints can be compared to identify people. Both children share some bands with each parent.

6h

6h describe how techniques such as DNA fingerprinting, genetic modifications, and chromosomal analysis are used to study the genomes of organisms

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Person A and person B have different numbers of repeated DNA sequences in their DNA.

C G G G G G G GC C C CT T TT A A A A G GG G G G GA AA AAG

Person A 4 repeats 3 repeats

= restriction site

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This DNA sequence of 33 bases can be repeated many times in a sample of a person’s DNA.

6 repeats 7 repeats

2 repeats 5 repeats

3 repeats 4 repeats

C G G G G G G GC C C CT T TT A A A A G GG G G G GA AA AAG

Person A 4 repeats 3 repeats

= restriction site

Person B

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Person B

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This DNA sequence of 33 bases can be repeated many times in a sample of a person’s DNA.

6 repeats 7 repeats

2 repeats 5 repeats

3 repeats 4 repeats

MAIN IDeA 6h

DNA fingerprinting is used for identification.DNA fingerprinting to identify people has become a reliable and widely used process since the 1990s. Why? The specific nucleotide sequences that are repeated can be found in everyone. More importantly, from one person to another, the number of repeat sequences can differ greatly, even among broth ers and sisters.

DNA Fingerprints and probabilityIdentification with DNA fingerprinting depends on probability. Suppose that 1 in every 500 people has three copies of the repeat at location A. This means any person has a 1-in-500 chance of having a matching DNA fingerprint for that region of a chromosome. By itself, the number of repeats in one location cannot be used for identification, because too many people would match. But then suppose that 1 in every 90 people has six copies of the repeat sequence at location B, and 1 in every 120 people has ten copies of the repeat sequence at location C. Individual probabilities are multiplied by each other to find the total probability. Therefore, when the three separate probabilities are multiplied, suddenly the chance that two people have the same DNA fingerprint is very small.

1 ___ 500 3 1 __ 90 3 1 ___ 120 5 1 _______ 5,400,000 5 1 chance in 5.4 million people

figure 3.2 DNA FingerprintingA DNA fingerprint shows differences in the number of repeats of certain DNA sequences.

Infer How would the DNA fingerprints change if a different restriction enzyme cut the DNA in the middle of one of the repeated DNA sequences? 6h

DNA fragments with different numbers of repeated DNA sequences show up as different bands on a gel.

A DNA fingerprint finds differences in DNA by separating the fragments on a gel.

CONNeCt tO

GeNOMeRecall from the chapter Meiosis and Mendel that a genome is the entire set of DNA in a cell. You will learn more about genome research in Section 5.

Chapter 9: Frontiers of Biotechnology  263

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Formative Assessment

ReVIeWING MAIN IDeAS

1. On what, in a person’s DNA, is a DNA fingerprint based?

2. Describe two ways in which DNA fingerprinting is used. 6h

CRItICAL tHINKING

3. Compare and Contrast How are DNA fingerprints and restriction maps similar? different? Explain. 6h

4. Synthesize Briefly describe how restriction enzymes, gel electropho-resis, and PCR are used in DNA fingerprinting. 6h

CONNeCt tO

MUtAtIONS5. Why might noncoding

regions of DNA outside of genes be more variable than coding regions of DNA?

Usually, DNA fingerprinting compares at least five regions of the genome. That way it is more certain that the pattern of DNA fragments in the fingerprint is unique. The more regions of DNA that are studied, the less likely it becomes that another person would have the same DNA fingerprint. For this reason, DNA fingerprinting is considered very reliable for identification purposes.

Uses of DNA FingerprintingDNA fingerprints are often used in legal cases. Because PCR can make a large sample of DNA even when there is a very small sample to start with, DNA fingerprints can be made from a few cells. Evidence, such as that shown in figure 3.3, can come from just a single drop of blood. Sometimes, DNA fingerprints are used against a suspect, but other times they are used to prove someone’s innocence. The Innocence Project at Benjamin Cardozo Law School in New York City has used DNA evidence to help free nearly 300 wrongfully convicted people. Through DNA fingerprinting, the Innocence Project showed that DNA

from those people did not match DNA from the crime scenes. Proving a person’s guilt through DNA fingerprinting is harder than proving a person’s innocence. For example, a DNA sample can become contaminated with other DNA if it is not handled carefully. Investigators must also consider the prob-ability that another person has the same DNA fingerprint and what chance is low enough to be acceptable. In fact, there is no legal standard for this prob-ability of a random DNA fingerprint match. Outside of the courtroom, DNA fingerprints can prove family relation-ships, such as paternity and the kinship necessary for immigration requests. Small samples and bone fragments can be sequenced to identify victims of catastrophes or resolve historical debates. DNA fingerprinting may also be used to study human migration patterns and genealogy. Genetic comparisons through DNA fingerprinting are used to study biodiversity and to locate genetically engineered crops. And, as you saw at the beginning of the chapter, DNA fingerprinting has even been used in disasters to identify the parents of lost children.

Summarize How does identification by DNA fingerprinting depend on probability?

figure 3.3 DNA collected at crime scenes is used as evidence in many legal cases.

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